Multilayered superconductive composites are known in the art to be useful as conductive devices in a wide variety of forms and applications. Such multilayered composites are typically comprised of combinations of vapor phase deposited contiguous thin films of superconductive materials with either conductive materials, insulators or both conductive materials and insulators.
Common multilayered superconductive composites are utilized as Josephson junctions, such as the trilayer devices described in C. T. Rogers, A. Inam, M. S. Hedge, B. Dutta, X. D. Wu, and T. Venkatesan, Appl. Phys. Lett., 55, Nov. 6, 1989, pages 2032-2034 and in K. Char, M. S. Colclough, T. H. Geballe, and K. E. Myers, Appl. Phys. Lett., 62, Jan. 11, 1993, pages 196-198. In typical Josephson devices the multilayers consist essentially of two superconductive layers bridged by a normal conductive layer (called an SNS device), with the optional presence of an insulator to confine the extent of the device. The junction is formed at the area where the superconductive layers overlap and are only separated by the normal conductive material.
Superconductive composites are also commonly utilized as SIS devices which consist essentially of two superconducting layers separated by a very thin insulator layer, thin enough to allow superconducting electrons to tunnel from one superconducting layer to the other through said insulator (typically 1 to 10 nm), with the optional presence of a thick insulator to confine the extent of the device. The junction is formed at the area where the superconductive layers overlap and are only separated by the very thin insulator layer.
There are other structures which may be utilized in the fabrication of Josephson devices, as described in U.S. Pat. No. 4,454,522 and U.S. Pat. No. 5,134,117. In these devices only one superconductive layer is used. The device is formed at a step-edge or at a naturally-occuring grain boundary. Grain boundary devices have been made utilizing thallium-containing superconductors as in R. H. Koch, W. J. Gallagher, B. Bumble, and W. Y. Lee, Appl. Phys. Lett., 54, Mar. 6, 1989, pages 951-953. However, these devices have several disadvantages as compared with multilayer devices: 1) lack of control over the critical device parameter, I.sub.c R.sub.N which is the product of the critical current of the junction I.sub.c and the normal state resistance of the junction R.sub.N, and, for grain boundary junctions, 2) lack of reproducability and manufacturability due to the fact that these devices rely on the formation of defects.
Superconductive composites are also commonly used in making devices such as multiturn flux transformers, i.e., antennas (B. Roas, G. Friedl, F. Bommel, G. Daalmans, and L. Schultz, IEEE Trans. on Appl. Super., 3, No. 1, 1992, pages 2442-2444) or, more generally in circuits which require two superconductive lines to cross without making electrical contact. A flux transformer in conjunction with a Superconducting Quantum Interference Devices (SQUID) comprise a superconducting magnetometer, a device useful in the detection of extremely small magnetic fields.
While prior junction devices are satisfactory for many applications, it has not been possible to utilize certain of the particularly desirable thallium-containing superconducting materials, which can permit operation at temperatures higher than 90.degree. K., in the preferred multilayer devices due to difficulty in fabrication of superconducting layers in the composite which have the proper stoichiometry to render said layers superconductive. Thallium oxide is a relatively volatile oxide component and tends to evaporate from the deposited film during processing and to react, i.e., form undesired compounds, with certain materials such as Al.sub.2 O.sub.3 and SrTiO.sub.3, which are commonly used as substrates or insulators in device configurations.
A successful method for depositing thallium-containing superconductive films per se is described in Myers et al., Appl. Phys. Lett., 65(4), 25 Jul. 1994, pages 490-492. The process is also described in detail in commonly assigned copending application Ser. No. 08/151,236, filed Nov. 12, 1993, now U.S. Pat. No. 5,386,606. This process provided a method for depositing single thallium-containing superconducting films on a substrate. A successful method for depositing a single insulating layer on top of a thallium-containing superconducting film is described in Face et al., Advances in Superconductivity VI, T. Fujita and Y. Shinohara, Eds., (New York: Springer-Verlag, Oct. 26-29, 1993) pages 863-868, but problems arose when attempts were made to create a composite structure involving deposition of a second superconductive layer. The insulator described by Face et al. cannot be deposited in the presence of thallium vapor and therefore must be deposited at a relatively low temperature, since otherwise there would be thallium evaporation from the first deposited thallium-containing layer. The structure of the insulator is compromised by the low deposition temperature. Multiple attempts to grow a second thallium-containing superconductor on top of the insulator failed.